There is a surprisingly simple explanation for Obama’s up-and-down performance as a candidate during his reelection grind in 2012, for those lackluster TV appearances, for that epic flop Oct. 3 on the Denver debate stage that might yet cost him his presidency on Tuesday.

Until the final sprint, hejustwasn’tthatintoit.

The key to understanding the Obama enigma of 2012, according to more than a dozen Obama associates interviewed by POLITICO during the campaign, is that the president enthusiastically approved the message of relentless attacks against Mitt Romney. But until the last week of the campaign — when optimism made a major comeback — Obama executed it mirthlessly and mechanically, at times reinforcing the “meh” vibe of his supportive but uninspired base.

Obama’s pollster, Joel Benenson, told him early on that hope and change couldn’t be recycled in a country enduring three years of grim recovery, and the campaign’s highly effective June and July anti-Romney blitz in battlegrounds was brutally effective. But the attack-first, hope-second strategy never quite suited Obama personally — in fact, it seemed to directly contradict his transformational, upbeat brand.

That unease with his own campaign not only drained energy from his reelection effort but could have nasty, knock-on consequences if he wins. A scorched-earth victor won’t do much to heal Washington’s divisions, with a probable GOP House and Democratic Senate set to be at loggerheads, regardless of who wins the presidency.

“I get why he had to do what he did. It was smart politically. But he’s become the embodiment of the partisanship he once decried,” said The New York Times’sDavid Brooks, one of the few journalists to strike up a real relationship with Obama during his first term. “The dissonance [between Obama in 2008 and 2012] is so obvious. … Can you think of a president who ran more different campaigns the first and second times? I’ve tried. I can’t.”

Obama officials say their boss had no choice but to run the campaign he did, given Romney’s fact-challenged attacks on Obama’s record. They point to key moments — from the campaign’s first ad to Obama’s convention speech to his positive closing argument at the second debate — to bolster their claim that his 2012 message isn’t quite as downcast as people think.

“Elections are always about choices, and it was our job to lay out that choice throughout the course of this campaign,” said Deputy Campaign Manager Stephanie Cutter. “But now is the time to make our closing argument, when people are making their final choices, and the president is making a very compelling and persuasive case to stick with him and finish what we started. And, he’s enjoying it.”

Even Brooks, a right-leaning centrist whose critique annoys Obama’s team, acknowledges that Obama’s 2008 theme needed a makeover for 2012, given the initial lack of enthusiasm of his base and the country’s larger anxieties about the possibility of a long, slow national decline.

But the absence of such a powerful motivating message has contributed to the sense, among political professionals and regular voters, that Obama downsized his own aspirations to hold onto power, something he railed against in 2008.

Clearly that perception had an impact on Obama’s attitude, aides privately acknowledge, which in turn allowed the charisma-challenged Romney to portray himself at times as the optimistic, hopeful candidate.

“[Obama] found his voice in 2008. That voice has, for the most part, been missing in 2012,” said CNN contributor David Gergen, who counseled presidents Gerald Ford, Ronald Reagan and Bill Clinton.

“This isn’t the kind of politics this man likes to practice. … Another factor is that he’s a moody person. Bill Clinton, for instance, was a much sunnier person. He let things roll off him more easily. You could see that during [Clinton’s] reelection campaign. … The clothes of this campaign, the negativity, don’t really fit” Obama.

Gergen said that Obama’s attitude might not cost him the election, but “he may have cost himself a mandate, an opportunity to clear the air in Washington” by letting Romney back into the race after the lackluster first debate. Obama’s surge in September, following the conventions, would have resulted in a bigger win than he could possibly achieve now.

His failure to nail down a larger, more durable lead in the Denver debate meant Obama had to stay personally negative longer — delaying the closing pivot to positivity he had looked forward to.

“Look, he knew what he had to do to win in 2012, and that involved making the sharp contrast between himself and Romney. He never hesitated on that — operationally,” said a Democrat close to Obama attuned to the president’s variable moods.

“But he’s a guy who feels a lot more personally comfortable making an optimistic case himself. The negative stuff doesn’t come naturally, and it really shows. … It’s clear, he just wasn’t into that as much when he had to carry that [negative] message himself. … And [he’s got] no poker face. None.”

Obama hasn’t been uniformly flat. A new POLITICO/George Washington University Battleground Tracking Poll shows him now neck and neck with Romney in terms of his voter enthusiasm, a sign that some of his ’08 magic might be back.

And his stump speech has always had its optimistic passages, always brimmed with his argument for fairness, especially on tax policy and entitlement reform, and always contained allusions to the spirit of ’08.

“If people ask you what this campaign is about, you tell them it’s still about hope, you tell them it’s still about change,” Obama shouted at the end of his kick0ff rally in Ohio six months ago — staring directly at 3,000 empty seats.

But a back-to-the-future theme has taken hold in the past 10 days, as if someone flicked the “hope” switch back on, as Obama himself shifts to a more upbeat closing argument while also throwing himself into the crisis of Hurricane Sandy.

That hasn’t stopped his campaign from churning out the negative ads, but he’s been scaling back or softening his own Romney hits. On Thursday, Obama’s first day back on the trail after the storm, he scarcely mentioned his opponent.

And he’s warmed to the idea of identifying himself as the “champion” of regular Americans, a phrase he wrote himself after struggling for months to find a way to meld his current campaign’s aggressiveness with some of the old 2008 uplift.

Obama biographer Jonathan Alter, who has been traveling with the president during the final week of barnstorming in the battleground Midwest, understands why Obama couldn’t rerun his 2008 campaign. But he thinks it was a major mistake to cede the optimistic high ground to Romney for much of the race.

“At this point, it’s a question of why he wasn’t making this argument, with this much passion, a lot earlier on. … I think he was so worried about over-promising, he wound up under-promising and under-performing,” Alter said after an Obama campaign stop in Ohio.

“I get where he was coming from. If you took a ‘Morning in America’ campaign you would have gotten hammered, but a hope-and-change message and more sober approach aren’t mutually exclusive,” he added. “They may have gone too far in the other direction. There weren’t enough aspirational ideas for a second term.”

Brooks agreed, adding, “You couldn’t do hope-and-change and Bain at the same time?”

Obama insiders think he’s finally hit on the right hybrid. The new Obama speech, with its explicit references to his promises of 2008, still contains its share of anti-Romney lines. But now they are in the context of a renewed Obama commitment to “change” — a challenge to Paul Ryan’s riff that Romney-Ryan is the new and improved “change and hope” ticket.

The address resonates more with grander themes of his election night address to 240,000 in Chicago four years ago than with the Big Bird, Seamus-on-the-roof and Bain attacks of 2012. And it’s given the candidate new spark — along with the fact that an exhausted Obama can now see the finish line.

“I’ve known him for 20 years. … I’ve never seen him more exhilarated than he is right now,” senior adviser David Axelrod told reporters traveling with the president in Ohio late last week.

“You can see in the speech he’s delivering … that this is coming from his loins,” he said.

Obama doesn’t own the franchise on incumbent ambivalence. Anyone who’s had hands on the controls, as a sitting president or as a vice president running from Air Force Two, feels the indignity of having to campaign again.

“These guys do get pissy. That happens to all presidents running for a second term, but it’s especially true for Obama, given the kind of campaign he ran the first time,” said Richard Ben Cramer, author of “What It Takes,” a landmark chronicle of the trials of the 1988 presidential candidates, including President George H.W. Bush, a diffident politician accustomed to the trappings of power.

“It’s very hard to top the presidency. … It’s really hard to get into the mode of, ‘Well, I have to talk to these guys in Medina, Ohio,’” he added. “For 3½ years, nobody tells the president, ‘You f—ed up,’ and all of a sudden people are telling him what to do. … So, I think Obama wants to be there, wants to be president, but he doesn’t necessarily want to do all the things he had to do to be there.”

That’s not to say Obama is ambivalent about the result of the election — he’s a ferocious competitor. Behind the scenes, insiders paint a portrait of a candidate deep-diving into polling reports on Air Force One and in Roosevelt Room strategy sessions, and who does, in fact, personally approve every message.

In speech-writing meetings, he’s often the one to suggest the addition of more optimistic, aspirational passages, aides say — even though he has frequently bowed to the counter-argument that he can prevail only by defining Romney in the most negative way possible.

The tension took its toll. Obama infamously told a Nevada campaign worker that his Denver debate prep was “a drag,” and aides say he was far more engaged during his daily economic and intelligence briefings than the atmosphere of “That was great, Mr. President, but…” at the cram sessions.

Obama’s field organization, micro-targeting operation and strategic focus on eroding Romney’s reputation in a half-dozen swing states have kept him in the race even when the candidate himself has sometimes “underperformed,” in the words of one former Clinton aide.

The reaction among the party faithful, and some of Obama’s own staff and volunteers, was rage, a sense that they were slaving 15-hour days, building a billion-dollar juggernaut — all for a man who hadn’t bothered to show up when it mattered most at the first debate.

“It’s not that he didn’t care, it’s that he was confused by the competing priorities,” said a Democrat close to Obama of his confounding performance in Denver. “That got him down.”

Reporters who have been with the president were less surprised by the flop. Obama’s performance on the trail has been so variable that reporters covering him seldom know quite what to expect. Unlike his former rival Hillary Clinton, who could campaign on three hours sleep, Obama needs his rest and often delivers his worst speeches when he doesn’t get enough shut-eye. The ’08 flashes of inspiration have come, but sporadically.

The day after he bombed in Denver, he arrived at a park early in the morning and electrified the audience, one hand in his pocket, the other one gripping the podium or sweeping off to the side at a 90-degree angle from his body, head craned into the bank of microphones — a gesture that signals to his staff that “he’s really into it.”

A few days later, during an appearance at Ohio State University, he seemed to be going through the motions and struggled to get through the canned Sesame Street gags packed into his stump speech to razz Romney over his threat to cut PBS funding.

“So for all you moms and kids out there, don’t worry, somebody’s finally getting tough on Big Bird,” Obama said, stumbling over the words. “Who knew that he was driving our deficit? So he decided we’re going after Big Bird, Elmo’s making a run for the border and Oscar’s hiding out in a trash can. Gov. Romney wants to let Wall Street run wild again, but he’s going to bring down the hammer on Sesame Street. Look, that is not leadership, that’s salesmanship. We can’t afford it.”

The contrast between that kind of message and the speech he delivered on Nov. 4, 2008, couldn’t have been more stark.

“If there is anyone out there who still doubts that America is a place where all things are possible, who still wonders if the dream of our founders is alive in our time, who still questions the power of our democracy, tonight is your answer,” Obama said then.

His personal affection for such language explains, in part, why Obama seems so thrilled to replace Big Bird with some of the aspirational touches of 2008. But there’s something new: humility.

“It started to click after Denver, when he began asking voters for their help, rather than telling them why they shouldn’t be so disappointed,” said one former Clinton aide who backs Obama.

The irony is that the perfect pitch for 2012 ultimately has been one from 2008 — not his own, but Hillary Clinton’s claim that her political “scars” from the 1990s made her tough enough to change Washington in the future.

“You know I’m willing to make tough decisions, even when they’re not politically convenient,” Obama says in his closing pitch. “I know what real change looks like, because I fought for it.